1. Field of Invention
The present invention relates generally to the field of laser sources using an external cavity and, more particularly, to methods and apparatus for external-cavity one-dimensional wavelength beam combining using two-dimensional laser sources.
2. Discussion of Related Art
High-efficiency multi-wavelength laser sources that use laser arrays are utilized for a variety of applications including materials processing, laser pumping and numerous medical procedures. External-cavity one-dimensional wavelength beam combining of diode laser arrays and two-dimensional diode stacks has been described in U.S. Pat. No. 6,327,292 as a technique to enhance the power and brightness of laser arrays and stacks.
Referring to FIG. 1A, there is illustrated an example of a wavelength beam combiner base-line architecture for external-cavity one-dimensional wavelength beam combining of two-dimensional laser stacks. FIG. 1A illustrates a closed-loop wavelength beam combining cavity. The cavity comprises a laser stack 110 which, in the illustrated example, includes a vertical stack of three laser diode bars, each bar comprising a plurality of laser diode elements, also referred to as emitters, to be combined. The cavity also comprises a cylindrical lens 120, diffraction grating 130, and a partially reflecting output coupler 140. The cylindrical lens 120 is placed at a distance equal to one focal length between the laser stack 110 and the diffraction grating 130. The cylindrical lens 120 converges the optical beams from the laser diode elements of each diode bar in the stack such that the beams are spatially overlapped, forming a region of overlap at the surface of the diffraction grating 130. The partially reflecting output coupler 140 is placed on the path of the first-order diffracted beams from the diffraction grating 130 and reflects a portion of each beam back toward the region of overlap, and the diffraction grating 130 then reflects light back to the laser stack 110. A resonant cavity is thereby formed between partially reflecting output coupler 140 and the laser diode elements of the laser stack 110. The partially reflecting output coupler 140 and the diffraction grating 130 thus provide feedback that forces each laser diode element in each respective diode bar to lase at a unique, but controlled, wavelength, and overlap the optical beams in the near field (at the output coupler 140) and far field. Thus, as shown in FIG. 1A, by properly arranging the cylindrical lens 120, diffraction grating 130, and output coupler 140, a single beam for each diode bar in the laser stack 110 can be produced.
In the example illustrated in FIG. 1A, the output beams 150 include three beams, one from each of the three diode bars in the laser stack 110, and each of the three output beams comprises the spatially overlapped optical beams from the laser diode elements making up the respective diode bar. Thus, wavelength beam combining is performed along the horizontal dimension of the laser stack 110. Stacking of multiple diode bars in the vertical dimension is for power scaling.
FIG. 1B illustrates an open-loop wavelength beam combining cavity. In the open-loop cavity, the laser elements are wavelength stabilized to a unique wavelength using a wavelength-chirped volume Bragg grating (VBG) 160. The volume Bragg grating 160 is placed as close as possible (e.g., about 1 mm) to the laser stack 110. The cylindrical lens 120 and the diffraction grating 130 match the wavelength spread of the volume Bragg grating 160. Again, wavelength beam combining of the laser elements is performed in the horizontal dimension, and vertical stacking of multiple diode bars in the vertical dimension is done for power scaling.